She quickly became one of several sexual slaves, or "sadakas" and was made to bear her master's children. She was subjected to regular beatings.
Now the former slave from Niger has won a landmark case against her government, which a regional West African court found had failed to protect her.
The court has ordered the government to pay her 10m CFA francs (£12,430; $19,750) in compensation.
"I will be able to build a house, raise animals and farm land to support my family. I will also be able to send my children to school so they can have the education I was never allowed as a slave," Ms Mani said afterwards.
The ruling could have broad implications for countries nearby where slavery is still practised, including Mauritania, Burkina Faso and Mali, according to observers.
Human rights organisations say more than 40,000 people are still in slavery in Niger, though the government says this figure is exaggerated.
Most live in conditions little changed over centuries, forced to look after animals or domestic work such as cooking and cleaning without pay.
Born into an established slave caste, they inherit a status from their mothers that it is almost impossible to shake off.
Romana Cacchioli, Africa Programme Coordinator for Anti-Slavery International, says this form of slavery began centuries ago when North African Berbers and Arabs raided the settlements of black Africans to the south and enslaved them.
Hadijatou Mani's case was different - she was sold by her Berber, Tuareg family to a master from the Hausa community.
She says he bought her for the equivalent of about $500 (£315).
"My master has four wives. We, the slaves, were doing all the housework like cooking, fetching water and firewood and working on farms," she told the BBC.
"I was beaten so many times I would run to my family. Then, after a day or two, I would be brought back."
But one day she heard that Niger had banned slavery, a decision that was announced in 2003.
"One of the anti-slavery activists from the Timidria association went to see the local chief who summoned my master.
"He was told that slavery had now been abolished, that he had to free his seven slaves, but he denied having seven slaves - he said he had only three, including myself.
"They told him if he loved us and we agreed, he could marry us. Otherwise he should just let us go. When he came back home he didn't tell us what happened, he just took us to another location so that we could not hear the news.
"After a while we came back, and that was when I heard that slavery no longer exists. I decided not to go back to my master but he kept going to the court saying that I am his wife."
In 2005, her master freed her and gave her a "liberation certificate", reports Anti-Slavery International, which helped her bring the case.
But when she left him and tried to marry another man, her "master" said they were married.
A local court found in favour of Ms Mani and she went ahead with her new wedding.
But this was then overturned on appeal and she was sentenced to six months in prison for bigamy.
She took her case to the Court of Justice of the West African regional body Ecowas earlier this year.
Ms Mani accused the government of Niger of failing to protect her from slavery, which was criminalised five years ago.
"I was wrongly jailed, not because of anything I did but because of slavery, and today there is no more slavery so I wanted the court to vindicate me, to give my rights which I was denied some four years ago, to compensate me," she said.
Ilguilas Weila, head of the local human rights group Timidria, said the situation in Niger had barely changed since the country announced that it was banning slavery.
"There has been a lack of political will," he said. "The law was only passed for Westerners. It was a charm-offensive aimed at those who were asking why slavery had not been made illegal."
Slaves are kept in humiliating and degrading conditions, he said. They can be beaten, sold, or given away as wedding presents.
"They wake up before their masters, and they are the last to go to sleep. During the day, the men look after the animals, the women collect water feed the family and gather wood."
"Almost the whole of the slave's day is spent working for their master."
Mr Weila said his group had estimated that in 2002 about 8% of the population in six of Niger's eight regions were living in slavery.
Ms Cacchioli says Anti-Slavery International has helped free about 80 women in Niger over the past five years.
She says that leaving their master is more difficult for women, as this also means abandoning any children she has had with him.
And the deeply-rooted practice has persisted in some neighbouring countries.
Ms Cacchioli says there are no reliable figures for the number of slaves in Mali.
Mauritania has also officially abolished slavery, but Anti-Slavery International says 18% of the population are estimated to be slaves.
The government there strongly disputes these figures.
Officials and some ordinary Mauritanians argue that it is difficult to define who is a slave - few records are kept, unlike during the trans-Atlantic slave trade.
They point out that abolishing slavery - and its scars - is not straightforward.
The practise is most widespread in remote, rural areas. Few have been to school and so they not be aware that slavery has been abolished.
And if they do manage to leave their masters, without training and land, they could just add to the ranks of the unemployed in the cities.